Polybutylene terephthalate resins (hereafter also referred to as “PBT resins”) have a high heat distortion temperature and exhibit excellent electrical properties, mechanical properties, weather resistance and chemical resistance and the like, and are consequently widely used as engineering plastics in a variety of applications, including electrical and electronic components, and vehicle components and the like.
Because PBT resins are thermoplastic resins, an injection molding method is generally used to obtain molded articles of the resin. An injection molding method is a method in which the melted resin composition is poured into the cavity of a mold that has been heated to a temperature no higher than the softening temperature of the resin, thereby solidifying the resin and molding the resin into the shape of the mold cavity. Following molding, the molded article is released from the mold, but if the molded article and the mold are stuck strongly together, then mold release can prove difficult, and the molded article can sometimes be destroyed or damaged during the release process. Accordingly, in injection molding methods, improving the mold releasability is extremely important. In order to improve the mold releasability, additives such as mold release agents and lubricants are typically added to the resin composition (for example, see Patent Documents 1 to 3).
On the other hand, in one example of an injection molding method, by pelletizing a resin composition containing the PBT resin and a lubricant, and feeding the resulting pellets into a hopper on the injection molding machine, the pellets pass through a cylinder supply unit, a plasticization unit and a measuring unit, and are then injected from a nozzle into the cavity of the mold. In this case, if the transport of the pellets through the supply unit or the melting of the pellets in the plasticization unit becomes unstable for some reason, then a problem arises in that fluctuations occur in the time required to measure the amount of resin required for a single shot in the measuring unit.
Patent Document 1 discloses a polyester resin composition comprising a thermoplastic polyester resin and a fatty acid ester. This resin composition enables improvements in the mold releasability and the measurement stability and the like. For the fatty acid ester, a compound obtained from a polyhydric alcohol selected from among glycerol and pentaerythritol, and a fatty acid having a carbon number of 12 or greater is used. However, when a fatty acid ester having a small hydroxyl value (for example, pentaerythritol tetraerythritol) is used as a simple substance, the measurement stability tends to deteriorate. Further, because the PBT resin has ester groups within the molecule, it has another shortcoming in that under conditions of high temperature and high humidity, a deterioration in the physical properties tends to occur due to hydrolysis. As a result, improvements are required in the hydrolysis resistance, but the hydrolysis resistance of the above resin composition cannot be said to be totally satisfactory.
Patent Document 2 discloses a styrene-based resin composition comprising: (a) a styrene-based resin, (b) a higher fatty acid ester or the like of pentaerythritol, (c) ethylenebisstearylamide, and (d) a low-molecular weight polyethylene. This resin composition enables an improvement in the mold releasability, but improvements in the measurement stability were not pursued.
Patent Document 3 discloses a resin composition comprising a specific polyester resin containing a PBT resin, and at least one lubricant selected from among specific phosphate esters, montan wax and polyethylene wax. This resin composition enables improvements in the mold releasability, the plate-out characteristics, and the calendering processability such as the drawdown characteristics. However, investigations by the inventors of the present invention revealed that the resin composition suffered from discoloration under high-temperature conditions.